The invention describes a method for preparing metal nanoparticles, and more particularly, to a method of making metal, or metal compound nanoparticles on a support within a zeolite matrix.
Metal nanoparticles have been shown to have utility as catalysts, where the size of the metal nanoparticles affect the catalytic activity. Small differences in particle size have been shown to affect catalytic activity for clusters of a few dozen atoms and smaller. It has been shown, for example, that the rate constant for the reaction with hydrogen of a cluster of 10 iron atoms is almost three orders of magnitude greater than that of either an 8- or a 17-atom cluster. The reactivity of clusters of 5 to 14 transition metal atoms with carbon monoxide was shown in an extensive study to vary by no more than factors of 2-3 as a function of cluster size and metal. Reducing the particle size, therefore, offers not only benefits in terms of an increase in catalyst dispersion (e.g., surface area available for reaction), but also potentially dramatic changes in the chemistry on the cluster surface. Furthermore, the size of the nanoparticles and nanoclusters can affect the stability of the nanocluster materials. For example, under conditions where bulk gold (Auo) is oxidized to Au2O3 (Au3+), clusters containing 55 gold atoms (1.4 nm diameter) were found to be highly stable compared to other clusters in the size range <1 to 7.9 nm.
Typically, metal nanoclusters are put on a support material when used as a catalyst. For example, noble-metal fuel cell catalysts are generally prepared by depositing metal nanoparticles or clusters onto a highly conductive support material, such as carbon. The conductivity of the support ensures that electron transfer efficiency is high.
One difficulty in catalysts used in hydrogen fuel cell electrodes is that high concentrations of platinum are generally required, 25-50 g per fuel cell stack for automotive applications. The high cost and scarce supplies of platinum are thus significant barriers toward the commercialization of fuel cell vehicles. Catalysts requiring platinum loadings of less than 5 g per vehicle or utilizing a more readily available active metal would boost the development of fuel cell powered vehicles.
Another problem associated with the preparation of such metal catalysts on a support, however, is the tendency of the metal clusters to aggregate during the thermal treatment required for activation, thus reducing the effective surface area for catalytic reaction and altering the chemistry of the metal surface. One way to reduce the aggregation behavior is to encapsulate the noble metal particles within a porous network, such as a zeolite, where cluster diffusion is reduced. Zeolite-supported metal clusters and particles are used in many commercial catalytic processes, including hydrotreating, hydrogenation/dehydrogenation, and environmental catalysts such as vehicle emission control. Zeolite-supported metal clusters, however, do not make good electro-active catalysts, since the zeolite matrix possesses relatively low conductivity.